The raf gene is an evolutionarily old gene and well conserved throughout mammals. At present, four v-raf-related genes are known in man and mouse: c-raf-1, A-raf-1, and their inactive pseudogenes. Here we describe attempts to identify further raf- related genes. Using A-raf as the probe for screening human cDNA libraries led to the isolation of four candidate clones. Their molecular characterization is currently underway. One seems to be highly homologous to A-raf, yet deviates in restriction pattern and preliminary nucleotide sequence. As judged from Southern blot hybridizations, the other clones show moderate raf homology. Two of them show identical length, restriction pattern, and nucleotide sequence (partially obtained). We then present evidence that S. cervisiae contains a raf-related gene(s). At present, we are cloning the gene(s) from yeast genomic and cDNA libraries. While, in general, yeast and mammalian proteins share extensive structural and functional similarities, the simplicity and experimental accessibility of yeast allow protein function studies which are severely impeded by the complex organization of mammalian cells. Thus, we want (1) to gain insight into functional properties of the raf gene products concerning interaction with ligands and regulation of kinase activity, and (2) to test whether the observation made in fibroblasts that ras function is dependent on raf for cellular growth control also applied to yeast. If so, a eukaryotic model system for studying transduction of growth-regulating signals may be developed.